This is my third Pokemon game, after finishing X and playing a bit of Black, and man, these games are just as entertaining as the first time I tried them.
I´m coming to Pokemon very late (I´m in my early 30s already), and even without any nostalgia from my youth I find them incredibly enjoyable.
I´m also stunned on how many levels people enjoy them. I stumbled across some guy hunting for shinies on YouTube some years ago, just a dude running three GBAs side by side, pushing buttons mechanically, and then suddenly screaming in a fit of elation when he got a slightly differently colored pony Pokemon.
Then there´s the endless walls of text on stuff like Smogon.com, super in-depth terms and discussions, which reminds me a lot of fighting game stuff like "frame data" or whatever. I just look at some of these articles and have absolutely no idea what´s going on - but this type of really in-depth gameplay analysis still fascinates me a lot.
I personally enjoy these games for checking out Pokemon designs (I mean, who DOESN´T love Spinda?), trying to get a feeling for strengths and weaknesses (it´s so nice to have an RPG that doesn´t just have fire/ice/volt and maybe light/dark) - basically just discovering these games and experimenting.
I´m trying to think of another series that has an even remotely comparable broad appeal to both the super casual and the extreme hardcore - maybe Smash Bros, but not quite.